I am moving from a 2010 Mac Pro to a 2015 (current) 27" iMac with a 2TB Fusion drive. I also have an external (OWC Thunderbay) external drive enclosure. The Mac Pro was configured with an internal 500GB SSD boot drive. I kept large files and those not needing fast loading on an internal 7200rpm SATA drive. I can imagine a variety of ways of configuring the iMac + Thunderbolt 2 SSD combination, and I wonder if others have experience that might be useful. Here are two I came up with... Use internal 2TB Fusion drive as the boot drive, and place files benefiting from quick loading on the SSD in the TB enclosure. Obviously, this means that many apps and files in my user folder will be on the slower part of the Fusion drive (the mechanical part). Use the 500GB SSD in the Thunderbay enclosure as the boot drive, and place large files and those not needing fast loading on the internal Fusion drive. In general, an SSD is much faster than a mechanical drive. But I don't know how to compare the relative speeds of a Fusion drive that is internal to an SSD that is external, but in a Thunderbolt enclosure (that enclosure will also have 3 mechanical drives in addition to the SSD). I'm not sure I can really test throughput with something like Black Magic Speed Test, because I have no way of kno wing whether when it tests the Fusion drive it is reading/writing the 128GB SSD in the iMac or the mechanical drive part of the Fusion drive, right? (FWIW, right now I have about 300GB on the Fusion drive, and I ran Black Magic Speed test, and the Fusion drive blew away the external SSD! The Fusion drive scored about 1300 read/525 write, while the external Thunderbolt SSD did 385 read/340 write. This surprised me. I also don't know what kind of mechanical drive is in the 27" iMac (7200 rpm? 5400?) nor the kind of interface it has (thus I don't know its xfr rate). Thoughts?